Semi-conductor



J. E. JACOBS SEMI-CONDUCTOR May 24, 1960 2 Sheets-Sheet 2 Q QSAIN1+ orWN: @t

NVENTOR- JOHN E. JACOBS ATTORNEY SEMI-CONDUCTOR John E. Jacobs, HalesCorners, Wis., assignor to General Electric Company, a corporation ofNew York Filed Nov. 25, 1955, Ser. No. 548,947

12 Claims. (Cl. 252-501) The present invention relates in general tophotosensitive semi-conductors, and has more particular reference tocurrent amplifying semi-conductor materials of unusual photosensiti-vityparticularly well suited for X-ray detection purposes, the presentapplication comprising a continuation in part of copending applicationsfor U.S. Letters Patent, Serial Numbers 228,333 and 269,276,respectively filed May 25, 1951, and January 31, 1952, both nowabandoned, on the inventions of John E. Jacobs relating tosemi-conductors.

The most widely known semi-conductors are those, such as selenium, whichare particularly responsive to visible light, or rays of wave lengthadjacent that of visible light in the light spectrum. Such commonlyknown semi-conductors are not sufliciently responsive to X-rays to allowthe useful application thereof to X-ray detecting purposes. The mannerof applying cadmium sulphide for the effective detection of X-rays, istaught in U.S. Letters Patent Number 2,706,790, which issued April 19,1955, on the invention of John E. Jacobs from an application for U.S.Letters Patent copending with the applications from which the subjectmatter hereof is derived. The impedance variation in response to X-rayirradiation ex'- hibited by heretofore available crystals of thesubstances mentioned is not at all uniform. Considerable difficulty,therefore, has been encountered in attempting to apply crystals of thenamed material for X-ray detection purposes, since the response of eachindividual crystal to X-ray irradiation is apt to vary from that ofother apparently identical crystals. It is thought that the irregularityof crystal response in the named X-ray sensitive semi-conductor materialis due, in part at least, to lattice irregularities or distortions inthe crystal structure, and to the occurrence in the crystals of varyingquantities of contaminant foreign materials. It is, therefore, animportant object of the present invention to provide semiconductivecrystals having substantially uniform response characteristics withrespect to variations in the impedance of the material when excited byX-rays impinging thereon.

Another important object is to process cadmium sulphide to increase itssemi-conductive photosensitivity not only to X-rays, but also to lightrays within a wide wave length band, including visible light rays; afurther object being to incorporate a sensitizing medium in themolecular lattice structure of the material. A l

Another important object is to apply indium or gallium as an activatingmedium, in the lattice structure of crystalline cadmium sulphide to thusgreatly alter and improve its light responsive characteristics; afurther object being to apply indium or gallium in trace quantities ofthe order of one part in one million, in the lattice structure ofhexagonal cadmium sulphide crystals, to thus render the samesemi-conductively photosensitive, especially to X-rays and similar raysof penetrating character.

Another important object is to provide cadmium sulphide as an activated,super-sensitive lightu responsive ttes Patent f semi-conductor for anyphotoelectric control purpose, and especially for X-ray detection.

The foregoing and numerous other important objects, advantages, andinherent functions of theinvention will become apparent as the same ismore fully understood from the following description, which, taken inconnection with the accompanying drawings, discloses a preferredembodiment of the invention.

Referring to the drawings:

Fig. 1 is a diagramatic showing of apparatus embodying a semi-conductorfor ray detecting purposes;

Figs. 2-5, inclusive, are graphicalcharts illustrating the performanceof cadmium sulphide activated as a semiconductor material in accordancewith the present invention;

Fig. 6 is a diagrammatic representation of a process for makinghexagonal cadmium sulphidepcrystals;

Fig. 7 is a sectional view taken through appaartus for producingcrystalline material in accordance with the present invention;

Fig. 8 is a sectional view taken substantially along the line 8 8 i'nFig. 7; and

Fig. 9 is an enlarged view of a portion of the structure shown in Fig.7.

Y To illustrate the invention, the drawings show a semiconductor element11, interconnected in a suitable electrical translation system 12,designed to measure the im pedance of the element 11 in terms ofelectrical power delivered to a load device 13 connected with the outputside of the system. The load 13, of course, may comprise any suitablemeans for the performance of any desired operation in response tochanges in the measured impedance ofthe element 11.

While any suitable or preferred translation system may be employed, thesame, as shown in Fig. 1 of the drawings, preferably comprises anelectronic amplifier including an amplifying tube 14 having an anodeplate 15, a vcathode 16, and a control grid `17, the plate 15 andcathode 16 being interconnected in an output circuit, including asuitable source of plate circuit power 18 and the operable device orload 13. The control grid 17 is interconnected in a control circuit inwhich the element 11 is also operatively connected, in order that thegrid 17 may be electrically energized in accordance with the transitoryimpedance values of the element 11.

As shown, the control circuit may comprise the element 11, a preferablyuni-directional power source 19, and a ballast or control resistor 20interconnected in series, 'so that electrical potential correspondingwith the impedance characteristics of the element 11 may be developed atthe opposite ends 21 and 22 of the resistor 20. The control grid 17 maybe connected with the control circuit at the connection point 21,preferably through a condenser 23, for filtering unidirectional voltagecomponents and allowing the application only of `fluctuating voltagecomponents on the grid 1K7. If it be desired to apply uni-directional aswell as uctuating voltage components, on the control grid 17, thecondenser 23 may be eliminated; and, if desired, means may besubstituted for excluding fluctuating voltage components while passingonly uni-directional voltage components to the grid, if it be desired tocontrol the load device 13 in response to such uni-directionalcomponents.

Means for applying a suitable bias between the cathode 16 and grid 17may also be provided, the same preferably comprising a suitable source24 of grid biasing power and a regulating resistor 25 interconnected inseries with the power source 24 between the cathode and the grid, theconnection point 22 of the control circuit being connected with the gridbias means, as at a connection point between the cathode 16 and theresistor 25.

The element 11 comprises crystalline cadmium sulphide activated byapplying atomic traces of indium or gallium, in quantities of the orderof one part in one million, in the molecular lattice structure of thematerial.

Activated hexagonal crystals of cadmium sulphide may be produced byvapor phase chemistry procedures, by mixing the activating `substance inits vapor stage with the vaporized constituent component of cadmiumsulphide, under controlled conditions, from which mingled vapor crystalsof activated cadmium sulphide may be deposited. The mingled vapors maybe created by heating cadmium sulphide together with an appropriatequantity of a salt of the activating metal, such as'the chloride ofindium orgallium, in a platinum boat, disposed in an end of a tubularglass retort, at a temperature of theorder of 1000 C. The mingled vaporsmay then pass to a cooler zone, as at the other vend of the retort,where crystals of the ydesired material may kgrow on the walls of theretort, at temperatures of the order lof 600 C. and lower. `Theresulting crystals will have atomic particles of indium or galliumdistributed, more or less at random, in the lattice structure thereof,which particles impart the desired photosensitive qualities in thematerial, such qualities being absent in material that is entirely freeof lattice impurities. Y

For the production of hexagonal cadmium sulphide' crystals the presentinvention contemplates the chemical reaction, as in a suitablecontainer, tank or vat 41, of preferably pure sodium thiosulphate(Na2S2O3) with preferably pure cadmium sulphate (CdSOt), in the presenceof water (H2O) as a carrying medium. The chemical reaction of suchsubstances results in the production of sodium sulphate (NazSOr),sulphur dioxide (SO2), water (H2O), and cadmium sulphide (CdS). Sodiumsulphate and sulphur dioxide, being soluble, become dissolved in thecarrying medium, while cadmium sulphate, being insoluble in the carryingmedium, is formed as a finely divided, powdery precipitate -42 in thevat 41.

` The powdery, dust-like precipitate 42 probably comprises cadmiumsulpbide in its cubic crystalline form, and the same may be separatedfrom the aqueous solution of sodium sulphate and sulphur dioxide byconventional filtration, after which the precipitate is preferablywashed, as with pure water as a washing medium, and dried afterseparation of the was-hing medium from the precipitate, as by`conventional ltration. The clean precipitate is then preferably formedinto pellets or briquets 43 by compressing the chemically preparedcadmium sulphide precipitate, in a suitable press under a pressure ofthe order of 10,000 pounds-per square inch. The resultingbriquetspreferably comprise cylindrical blocks having a diameter of the order of11/2 inches and a height of the order of 1/2 inch. A number of theseblocks may then be evaporated `in an atmosphere of hydrogen sulphide toproduce cadmium sulphide vapor, from which hexagonal cadmium sulphidecrystals may be condensed, in a manner hereinafter described.

The evaporation of the cadmium sulphide briquets 43, and subsequentcondensation of hexagonal cadmium sulphide crystals from the resultantcadmium sulphide Vapor, may be, and preferably is, accomplished incondenser apparatus 44'comprising an elongate sleeve-like housing 45.The housing 45 may comprise a tube of quartz glass having length oftheorder of 5 feet and internal diameter of the order of 3 inches.Suitable support means, of course, may be provided for mounting the tubein substantially horizontal operating position. The tube may be providedwith suitable end closure means, such as corks or Stoppers 46, 46',carrying inlet and outlet conduits 47, 47 sealed therein and openingtherethrough. The inlet conduit 47 may be connected, as through asuitable control valve 48, with a source of hydrogen sulphide gas, such.source preferably delivering hydrogen sulphide gas containing tracequantities of hydrochloric acidvapor. The conventional Kipp gen- 45 fromthe inlet conduit 47 to the outlet conduit 47.

'Ihe major portions of the housing 45 may be exposed to normalatmospheric temperature. Suitable heating means, such as an electricalheating coil 50, may be provided to heat a portion of the housing 45 inorder to constitute a hot zone in the housing, adjacent the inlet endthereof, at a temperature in excess of the vaporizing temperature ofcadmium sulphide. The surrounding atmosphere will cool the housing 45 onopposite sides of such hot zone. The heater 50 is preferably spaced fromthe inlet end of the housing a distance of the order of 12 inches, suchthat circumambient air will maintain the inlet end of the housing at atemperature sutiiciently low to avoid causing deterioration of theclosure means 46. It will be noted, also, that the wall temperature ofthe housing 45, due to the cooling effect of circumambient air, willdecrease progressively from the hot zone toward the discharge end of thehousing, the housing being of such length that, when the equipment is inoperation, its end portions at and adjacent the discharge end of thehousing will be substantially at atmospheric temperature.

A supply of briquets 43 may be loaded in a porcelain boat 51 andintroduced into the sleeve-like housing 45 through the inlet endthereof, as by removal of the closure means 46, the boat 51 containingits charge of briquets 43 being placed in the housing 45 Within the hotzone thereof. After the boat 5l and its contents has been placed in thehousing, the closure means 46 may be replaced in sealed condition on theend of the housing. The valve 48 may be opened to deliver hydrogensulphide carrying traces of hydrochloric acid vapor into the housing atthe inlet end thereof, and the fan device 49 may be set in operation toprogress the hydrogen sulphide and hydrochloric acid gases, togetherwith the cadmium sulphide vapor evolved from the briquets 43, throughthe housing at a relatively low rate of speed which may be determined bycontrolling the speed of operation of the fan device 49.

The apparatus may be maintanedin operation over an extended period ofthe order of 5 days, moreor less, during which period hexagonal cadmiumsulphide crystals 52 will become deposited and grow upon the inner wallsurfaces of the housing 45 between the hot zone and the discharge end ofthe housing. After the inner walls of the housing shall have thus becomecoated with crystals 52, the housing may be cooled by disabling theheater 50 and allowing the cooling to be accomplished slowly by actionof thecircumambient atmosphere for a period of the order of 24 hours,after which the crystals may be removed,r as by scraping the same, fromthe inner walls of the housing 45, by removing the closure means 46' toallow access to the housing 45.

In order to introduce an activator substance, such as indium or gallium,into the crystal material thus deposited by condensation upon the innerwalls of the housing 45, measured quantities of a suitable salt of theactivating substance may be uniformly mixed with the cadmium sulphideprecipitate 42 after filtration, washing and drying thereof, and beforethe formation of the same into briquets 43. To this end, the chloridesof gallium or indium may be uniformly mixed with the powdery cadmiumsulphide precipitate 42 in precisely measured quantities, whereby toobtain a desired atomic dispersion of the selected metallic activatorsubstance or substances in the resulting crystals 52.

The foregoing procedure results in the production of crystals havingsubstantially uniform X-ray responsive impedance variationcharacteristics. The procedure is not at all critical and is notaffected by variations in atmospheric conditions' from day to day, ashasbeen found to be the case with other attempted methods for thesynthesis of X-ray sensitive semi-conductors.

Crystals of cadmium sulphide activated in accordance with the teachingsof the preesnt invention show particularly uniform semi-conductivecharacteristics, such uniformity being accomplished by controlling theamount of activating substance in the lattice structure of the resultingcrystals. The desired crystals may thus be produced by introducing adesired proportion of activating substance, in its vapor stage, with themingled vapors from which the resulting crystals are produced. Suchactivated crystals are thought to contain atoms of the activatingsubstance, more or less uniformly distributed throughout the latticestructure of the crystalline material, thereby forming electron donorcenters lwhich serve to constitute the crystal as a photosensitivesemi-conductor, having current amplifying characteristics.

Satisfactory or optimum results are obtained when indium or gallium ispresent, in the vapor mixture from which the activated crystals aregrown, in amounts not exceeding 0.01% by weight of cadmium vapor in themixture produced. Such a result may be attained by evaporating a mixtureof cadmium sulphide and indium chloride in the proportions of 10,000 to1.5, by Weight. Where the activating metal is gallium, optimum resultsmay be attained by evaporating cadmium sulphide mixed in the proportionsof 10,000 to 2 by weight. By reducing the amount of activating metalsalt in the evaporable mixture, the resistivity of the semi-conductormay be correspondingly increased to any desired extent. In crystalsproduced from vapor containing indium or gallium in quantitiesappreciably exceeding 0.01% by weight of the cadmium component, the darkcurrent, that is, the current which flows through the semi-conductor inthe absence of irradiation, is unduly large, and the ability of thecrystal to `distinguish sharply between variations in light intensity iscorrespondingly impaired. This impairment of discriminating sensitivity,of course, is the result of reduction of the inherent electricalresistivity of the crystal due to the presence of excess quantities ofthe activating metal in the crystal.

When a crystal 11 of cadmium sulphide, activated with indium or galliumas herein disclosed, is exposed to X-rays emanating as from a ray source26, the impedance of the crystal changes substantially in proportion tothe intensity of impinging X-rays. Where the applied rays are ofpulsating character, the impedance change in the crystal preciselyfollows the pulsations of the impinging rays and consequentlyestablishes a corresponding pulsating voltage across the resistor 20,which, being applied to the control grid 17, produces correspondingpower pulsations for application to the load device 13.

X-rays produced by operation of the usual X-ray generating tubes,electrically excited by alternating current power at 60 cycles, compriseX-ray energy pulsations at a frequency corresponding with that of theenergizing power applied for the operation of the ray generator, X-raysof uniform, non-pulsating character may, of course, be produced andapplied upon the crystal 11, in which case the voltage developed acrossthe resistor will be of uni-directional character. Consequently, thetranslation system 12 should be designed to measure the magnitude eitherof the unidirectional impedance of the crystal or the iiuctuatingimpedance thereof, depending upon the uni-directional or fluctuatingcharacter of the impinging rays.

Semi-conductors of the character herein contemplated exhibit impedancechanges when exposed to visible light rays, as from a light source 27,and the extent of such visible light induced impedance change is inproportion to the intensity of rays impinging on the crystal from thesource 27. Accordingly, when a crystal is exposed to rays to which it issensitive, from a source other than the X-ray source 26, the voltageavailable at the connection points 21 and 22 may contain componentswhich correspond with t-he impedance value of the crystal, determined bythe light rays from such other source as well as componentscorresponding with the rays emanating from the X-ray source 26. If therays impinging on the crystal from the source 27 are -of uniformintensity, the corresponding voltage component across the resistor 20will be uniform. Where the impinging X-rays are of fluctuatingcharacter, the same may be applied through the condenser 23 for thecontrol of the amplifier 14, while the uniform voltage componentestablished by illumination of the crystal from the source 27, atuniform intensity, as well as the uni-directional X-ray induced voltagecomponent, will be excluded from the amplifier system by the action ofthe condenser 23.

The present invention, of course, is not necessarily limited to theexcitation of the crystal 11 by visible light or other rays from asource 27 and by pulsating X-rays from the source 26, but, in itsbroadest aspects, the invention applies to the excitation of the crystal11 by means of visible light or by means of X-rays or both, and whetheror not the light rays or the X-rays pulsate, there being many possibleadvantageous applications involving the excitation of the crystal eitherby X-rays or by other light rays including visible light, Where eitherthe light rays or X-rays are of uniform or of pulsating intensitycharacter.

The detection characteristics of the crystals do not alter upon exposurethereof to X-rays and other light rays. In this connection, theperformance of cadmium sulphide crystals of the so-called beta orhexagonal form, including standard crystals containing usual latticeimpurities, and crystals activated with indium in accordance with thepresent invention, has been investigated, using X-rays, having wavelength of 1.54 Angstroms, for crystal excitation. The X-ray beam thusapplied to the examined crystals was of pulsating character at afrequency of 60 cycles. To determine its impedance responsecharacteristics, a suitable uni-directional electromotive force, of thesort supplied by the source 19 in Fig. 1, may be applied to the crystaland the resultant current liow therethrough accurately measured. Theresulting crystal current obtained in response to pulsating X-rayirradiation of the crystal was found to contain an alternating as wellas a uni-directional component. The uni-directional component was foundto vary substantially linearly with the intensity of incident X-rays,the alternating component substantially as the square of incident X-rayintensity. This phenomenon is eX-plainable upon the theory that thealternating component is proportional to the rate of recombination ofelectrons in the effective conduction band or zone of the irradiatedcrystal.

The magnitude of the uni-directional component of crystal current wasfound to be approximately 10,000 times that of the alternating componentat intensities of approximately quanta/second, in standard cadmiumsulphide crystals containing natural impurities as activating media inthe crystal structure, as well as in crystals activated with indium orgallium in accordance with the present invention. The ratio was found tobe somewhat variable,'each crystal having its own characteristic ratio.At higher incident intensities the ratio diminishes, and it is supposedthat the ratio approaches unity in response to progressively increasingray intensity.

In standard cadmium sulphide crystals, the time lag before crystalcurrent reaches a maximum value following initial application of theX-ray beam is of the order of several minutes so far as theuni-directional crystal current component is concerned, but is of theorder of 1/6 of a second for the alternating component, in the samecrystal. Cadmium sulphide crystals activated with indium or galliumexhibit time lag characteristics of the order of 5 minutes and 1/asecond, respectively, for the uni-directional and iiuctuating componentsof crystal current.

Crystal sensitivity, whichmay be shown as the ratio of measured outputcurrent to incident X-ray intensity, varies from one crystal to the nextin accordance with the nature and extent of activating foreign matter inthe lattice structure of-the crystals. Under comparable test conditions,the magnitude of the X-ray induced current iiow,V both fluctuating anduni-directional, in standard cadmium sulphide crystals, as compared withcurrent ow in crystals activated with indium or gallium, is as one is toone hundred. These characteristics are illustrated in the graphscomprising Figs. 2-5. The curves shown in Figs. 2 and 4 illustratecrystal response data obtained, under like test conditions, byphotographing, as on an electron oscilloscope, the uctuating componentof crystal current immediately following the application of X-rays tothe crystals. The curve 2S in Fig. 2 shows the approximate responseobtained from a standard CdS crystal. The curve 29 in Fig. 4 shows thecorresponding response obtained for crystals activated with indium orgallium. The curves 32 and 33 in Figs. 3 and 5 respectively illustratethe values of uni-directional crystal current immediately followingX-ray application to a standard crystal and to a crystal activated withindium or gallium. The curves 32' and 33 illustrate the decline ofuni-directional crystal current component from the maximum valuefollowing discontinuation of X-ray application to the crystals. Upontermination of X-ray irradiation on the crystals, ow of uctuatingcrystal current ceases immediately at the conclusion of the cycle ofcrystal current flow then in being.

It will be noted that in standard as` Well as in crystals activated withindium or gallium the time lag required for the uni-directional currentcomponent to reach a maximum value is many time that within which thefluctuating current component of the crystal reaches its maximum value.This phenomenon may be explained upon the theory that the uctuatingcomponent is a measure of the change in the number of electrons presentin the conduction band of the crystal, yas a function of time.Accordingly, employment of the uctuating component of crystal current,to the exclusion of the unidirectional component, wll permit the almostinstantaneous measurement of crystal current for the determination ofX-ray intensity, thus avoiding the more extended delay necessary toachieve a stable condition in using the uni-directional currentcomponent. Measurement of the fluctuating current component only permitsthe advantageous use of high gain iluctuating current amplifiers .in thetranslation system, in the interests of effective instrumentation.

In Ithis connection, it will be noted that crystals activated withindium or gallium have time lag response characteristics comparable tostandard crystals, the time lag response of indium activated cadmiumsulphide being substantially slower than that of the same material whenactivated with other metals, such as copper, Vsilver and manganese. As aconsequence, indium activated cadmium sulphide crystals are lessdesirable for ultra high= speed detecting purposes, than crystals-activated with copper, silver or manganese, but, because of therelatively much larger values of crystal current obtainable in indium orgallium activated crystals, they are very much more sensitive thancrystals activated with other materials, and may be employed toadvantage Wherever speed of response is a negligible factor. Indium orgallium activated crystals, because of extreme sensitivity, areespecially suitable for use in visible light ray detection.

Employment of the alternating component of crystal current, for X-raydetection purposes, permits the application of secondary illumination asa light bias t the crystal for increasing the X-ray sensitivity thereof.When such a light bias is directed on a crystal, sensitive to the biasirradiation, While simultaneously irradiated with pulsating X-rays, boththe uni-directional and uctuating components of X-ray induced crystalcurrent are increased by a factor of the order of l0, as compared withsuch X-ray induced components in the absence of the light bias. Eachdiierent kind of crystal may have a corresponding optimum light biasWave lengthpproducing maximum response, the response being reduced byvariation of the light bias wave length above or below the optimum Wavelength value characteristic of the crystal. For cadmium sulphidecrystals, however, regardless of the manner of activation, the optimumsensitizing effect is obtained in response to light bias having a wavelength of the order of 5200 Angstroms.

It is thought that the invention and its numerous attendant advantageswill be fully understood from the foregoing description, and it isobvious that numerous changes may be made in the form, construction andarrangement of the several parts without departing from the spirit orscope of the invention, or sacrificing any of its attendant advantages,the form herein disclosed being a preferred embodiment for the `purposeof illustrating the invention.

The invention is hereby claimed as follows:

l. A photoconductive semiconductor consisting essentially of cadmiumsulphide activated with a metal of the class consisting of Vindium andgallium.

2. A photoconductive semiconductor consistingk essentially of cadmiumsulphide in its hexagonal crystalline form and activated with indium.

3. VA photoconductive semiconductor consisting essentially of `cadmiumsulphide in its hexagonal crystalline form and activated with gallium.

4. A photosensitive semi-conductor, consisting essentially of a crystalof cadmium sulphide having atomic particles of a metal of the classconsisting of indium and gallium dispersed in the lattice structure ofthe crystal to form electron donor centers therein.

5. A photosensitive semi-conductor, consisting essentially of a crystalof cadmium sulphide having atomic particles of a metal of the classconsisting of indium and gallium dispersed in the lattice structure ofthe crystal, in trace quantities of the order of one part in onemillion, to form electron donor centers therein.

6. A photoconductive semiconductor consisting essentially of cadmiumsulphide condensed in its hexagonal crystalline form from a vaporproduced by the evaporation of the constituents of cadmium sulphide anda salt of an activating metal of the class consisting of indium andgallium in proportions producing cadmium and the activating metal in thevapor in the relative proportions of 10,000 to 1 by weight.

7. A photoconductive semiconductor consisting essentially of cadmiumsulphide condensed in its hexagonal crystalline form from vaporcontaining its constituent elements together with traces of anactivating metal selected from the class consisting of indium andgallium in quantities of the order of 0.01% by Weight.

8. A photoconductive semiconductor consisting essentially of cadmiumsulphide in its hexagonal crystalline form containing atomic particlesof gallium dispersed in the molecular lattice structure of the crystalto form electron donor centers therein. n

9. A photoconductive semiconductor consisting essentially of cadmiumsulphide in its hexagonal crystalline form containing atomic particlesof gallium dispersed in the molecular lattice structure of the crystalin quantity of the order of one part gallium to one million partscadmium sulphide.

10. A photoconductive semiconductor consisting essentially of cadmiumsulphide condensed in its hexagonal crystalline form from vaporcontaining its constituent elements together with traces of gallium inquantities of the order of 0.01 percent by weight of the cadmiumconstituent.

11. A photoconductive semiconductor consisting essentially of cadmiumsulphide condensed in its hexagonal l crystalline form from a vaporproduced by the evaporation of the constituents of cadmium sulphide anda salt 9 10 of gallium in proportions producing cadmium and gal-2,623,857 Kroger et al. Dec. 30, 1952 lium in the vapor in the relativeproportion of 10,000 to 1 2,623,859 Kroger et al Dec. 30, 1952 byweight. 2,727,866 Larach Dec. 20, 1955 12. A photosensitivesemi-conductor consisting es- 2,810,052 Bube et al. Oct. 15, 1957sentially of gallium activated cadmium sulphide con- 5 densed fromvapors containing not more than one part OTHER REFERENCES of ganium to10,000 parts of cadmium by Weigm 39verenz: Luminescence of Solids,January 1950, p. References Cited in the le of this patent Academie desSciences Comptes Rendus, vol. 230, pp.

947-949, March 1950, Veith. UNITED STATES PATENTS 10 Smith: Phys. Rev.,Mar. 15, 1955, vol. 97, No. 6, p. 2,600,579 Ruedy June 17, 1952 1526.

6. A PHOTOCONDUCTIVE SEMICONDUCTOR CONSISTING ESSENTIALLY OF CADMIUMSULPHIDE CONDENSED IN ITS HEXAGONAL CRYSTALLINE FORM FORM A VAPORPRODUCED BY THE EVAPORATION OF THE CONSTITUENTS OF CADMIUM SULPHIDE ANDA SALT OF AN ACTIVATING METAL OF THE CLASS CONSISTING OF INDIUM ANDGALLIUM IN PROPORTIONS PRODUCING CADMIUM AND THE ACTIVATING METAL IN THEVAPOR IN THE RELATIVE PROPORTIONS OF 10,000 TO 1 BY WEIGHT.